The present invention relates to a transformer, and more particularly to a transformer adapted to be used as an ignition coil for supplying high voltage to an ignition plug.
As well known in the art, a conventional transformer is generally constructed by assembling two or more coils with a core of types such as EE, EI, UU and UI forming a closed magnetic circuit. Referring to FIG. 8 which is a cross section of the conventional transformer for outputting high voltage, the transformer includes E-cores 11 and 12 formed of a magnetic material such as ferrite, a low-voltage input coil 13, a high-voltage output coil 14, an insulating cover 15 entirely covering both the coils, a bobbin 16 on which the low-voltage input coil 13 is wound, and a bobbin 17 on which the high-voltage output coil 14 is wound. The bobbin 17 is formed with a plurality of flange portions 171 axially spaced a suitable distance to define a plurality of coil winding sections where the high-voltage output coil 14 is continuously wound. The bobbin 16 is coaxially inserted into an inner-diameter hole 172 of the bobbin 17, and the cores 11 and 12 of EE-, EI-, UU- or UI-type are inserted into an inner-diameter hole 161 of the bobbin 16 from axially opposite sides, thus forming a closed magnetic circuit.
While the invention has been particularly shown and described in reference to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the spirit and scope of the invention.
In the conventional transformer as mentioned above, the closed magnetic circuit is formed by the cores 11 and 12 of EE-, EI-, UU- or UI-type, magnetic connection between both the coils 13 and 14 is high. However, there exist the following problems.
(a) As the cores 11 and 12 are disposed outside the insulating cover 15, the general construction is enlarged to cause the difficulty in making the transformer compact, and the external shape is complicated. This type of transformer is used as an ignition coil for supplying high voltage to an ignition plug for an automobile, for example. Therefore, it is important to make the transformer compact and simlify the shape thereof in applying the transformer to the ignition coil.
(b) To make the transformer compact and simplify the shape thereof, there has been used an I-core forming an open magnetic circuit. However, the magnetic connection between the coils is decreased to reduce the efficiency of the transformer, resulting in a reduction in operation speed.
In a plug ignition circuit for an internal combustion engine, there is generated electromagnetic wave noise from an ignition plug device or the like due to spark discharge. Further, there are also generated magnetic noise, electrostatic noise and the like from the transformer in the ignition plug device. Therefore, it is necessary to suppress such noises. In a conventional ignition plug device, the generation of such noises is suppressed normally by interposing a resistor element for limiting a change in current between the ignition plug in the plug ignition circuit and the high-voltage output coil, or by using another type of ignition plug incorporating a resistor.
However, in the case of applying the transformer to the ignition plug device for the internal combustion engine, it is common that there hardly exists a sufficient space for installing the ignition plug device. Therefore, it is necessary to make the ignition plug device compact and simplify the shape thereof.
FIG. 9 shows the transformer shown in FIG. 8 with an ignition plug 118 connected thereto. The transformer is provided with a connector 119 for the connection with the ignition plug 118 in a direction perpendicular to an axial direction of the bobbin 117. Accordingly, the ignition plug 118 projects from the transformer at right angles to the axial direction of the bobbin 117.
As previously mentioned, this type of transformer is used as an ignition coil or the like for supplying high voltage to an ignition plug for an automobile, and it is common that there hardly exists a sufficient space for installing the transformer. Therefore, it is significant to make the transformer compact and simplify the shape thereof. However, as the ignition plug 118 is connected to the connector 119 at right angles to the axis of the bobbins 116 and 117 on which the coils 113 and 114 are wound, the external shape is complicated, and an occupied area of the ignition plug device is enlarged. Further, the ignition plug device cannot be easily installed.
FIGS. 10 to 12 are cross sections of an essential part of some conventional coil devices to be used for the transformer as mentioned above. The coil devices commonly include a core 11, a low-voltage input coil 12, a high-voltage output coil 13, a bobbin 14 on which the low-voltage input coil 12 is wound, and a bobbin 15 on which the high-voltage output coil 13 is wound. The bobbin 15 is formed with a plurality of flange portions 151 axially spaced a suitable distance to define a plurality of coil winding sections 152 where the high-voltage output coil 13 is continuously wound. The high-voltage output coil 13 is wound on the bobbin 15 from one axial end A to the other axial end B in the direction of arrow a. Therefore, the voltage is low at the one axial end A, and it is high at the other aixal end B. The bobbin 14 on which the low-voltage input coil 12 is wound is coaxially inserted into an inner-diameter hole 153 of the bobbin 15 on which the high-voltage output coil 13 is wound. The core 11 is inserted into an inner-diameter hole 41 of the bobbin 14.
In the coil structure as mentioned above, a sufficient insulating distance must be defined between the low-voltage input coil 12 and the high-voltage output coil 13 particularly at the other axial end B on the high voltage side of the high-voltage output coil 13. One of the measures for defining such a sufficient insulating distance is provided by the structure shown in FIG. 10. That is, a uniform insulating distance d.sub.1 is defined over the substantially entire axial length of the coil device between the low-voltage input coil 12 and the high-voltage output coil 13. Another measure is provided by the structure shown in FIG. 11. That is, the depth of the coil winding sections 152 of the bobbin 15 gradually decreases from the low voltage side to the high voltage side, so that the inner surface of the inner-diameter hole 153 of the bobbin 15 is negatively tapered, and accordingly the insulating distance gradually increases from the low voltage side to the high voltage side. A further measure is provided by the structure shown in FIG. 12, wherein the depth of the coil winding sections 152 of the bobbin 15 gradually decreases from the low voltage side to the high voltage side in the same manner as the structure of FIG. 11, and the wall thickness of the bobbin 15 gradually increases toward the high voltage side, so that the inner surface of the inner-diameter hole 153 is formed into a straight surface not tapered.
However, the coil devices as mentioned above still includes the following problems. In the coil device shown in FIG. 10, a relatively large insulating distance defined on the high voltage side is provided on the low voltage side where such a large insulating distance is not so required. As a result, the magnetic connection between the low-voltage input coil 12 and the high-voltage output coil 13 is reduced, and the general structure is enlarged. Since this type of coil device is importantly applied to an ignition coil for an internal combustion engine with an installation space limited, such a large construction of the coil device does not meet the requirement for the application to the ignition coil.
Further, in the coil devices shown in FIGS. 11 and 12, the shape of the bobbin 15 is complicated to cause an increase in cost because the bottoms of the coil winding sections 152 must be tapered.